1. Field of the Invention
The present invention relates to a tunnel junction sensor with an antiparallel (AP) coupled flux guide wherein the flux guide does not require stabilization by hard bias layers.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. When the slider rides on the air bearing the write and read heads are employed for writing magnetic impressions to and reading magnetic signal fields from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a tunnel junction sensor for sensing the magnetic signal fields from the rotating magnetic disk. The sensor includes an insulative tunneling or barrier layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90xc2x0 to an air bearing surface (ABS) wherein the ABS is an exposed surface of the sensor that faces the rotating disk. The tunnel junction sensor is located between ferromagnetic first and second shield layers. First and second leads, which may be the first and second shield layers, are connected to the tunnel junction sensor for conducting a sense current therethrough. The sense current is conducted perpendicular to the major film planes (CPP) of the sensor as contrasted to a spin valve sensor where the sense current is conducted parallel to the major film planes (CIP) of the spin valve sensor. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, is when the sense current is conducted through the sensor without magnetic field signals from the rotating magnetic disk.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the tunnel junction sensor to the sense current (IS) is at a minimum and when their magnetic moments are antiparallel the resistance of the tunnel junction sensor to the sense current (IS) is at a maximum. Changes in resistance of the tunnel junction sensor is a function of cos xcex8, where xcex8 is the angle between the magnetic moments of the pinned and free layers. When the sense current (IS) is conducted through the tunnel junction sensor, resistance changes, due to signal fields from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the tunnel junction sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the tunnel junction sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the tunnel junction sensor at minimum resistance. The dr/R of a tunnel junction sensor can be on the order of 40% as compared to 10% for a spin valve sensor.
The first and second shield layers may engage the bottom and the top respectively of the tunnel junction sensor so that the first and second shield layers serve as leads for conducting the sense current Is through the tunnel junction sensor perpendicular to the major planes of the layers of the tunnel junction sensor.
The tunnel junction sensor has first and second side surfaces which are normal to the ABS. First and second hard bias layers abut the first and second side surfaces respectively of the tunnel junction sensor for longitudinally biasing the magnetic domains of the free layer. This longitudinal biasing maintains the magnetic moment of the free layer parallel to the ABS when the read head is in a quiescent condition.
Magnetic head assemblies, wherein each magnetic head assembly includes a read head and a write head combination, are constructed in rows and columns on a wafer. After completion at the wafer level, the wafer is diced into rows of magnetic head assemblies and each row is lapped by a grinding process to lap the row to a predetermined air bearing surface (ABS). In a typical tunnel junction read head all of the layers are exposed at the ABS, namely first edges of each of the first shield layer, the seed layer, the free layer, the barrier layer, the pinned layer, the pinning layer and the second shield layer. Opposite edges of these layers are recessed in the head. The barrier layer is a very thin layer, on the order of 20 xc3x85, which places the free and pinned layers very close to one another at the ABS. When a row of magnetic head assemblies is lapped there is a high risk of magnetic material from the free and pinned layers being smeared across the ABS to cause a short therebetween. Accordingly, there is a strong-felt need to construct magnetic head assemblies with tunnel junction heads without the risk of shorting between the free and pinned layers at the ABS due to lapping.
A scheme for preventing shorts across the barrier layer of the tunnel junction sensor is to recess the tunnel junction sensor within the head and provide a flux guide between the ABS and the sensor for guiding flux signals from the rotating magnetic disk. Typically, the ferromagnetic material of the flux guide is required to be stabilized by hard bias layers on each side of the flux guide. With track widths of 1 xcexcm or more this stabilization of the flux guide has been acceptable. However, with submicron track widths, such as 0.1 xcexcm to 0.2 xcexcm, the hard biasing of the flux guide renders the magnetization of the flux guide too stiff to adequately respond to flux signals from the rotating magnetic disk. The reason for this is because flux guides, regardless of the track width, are magnetically stiffened about 0.1 xcexcm on each side of the flux guide by the hard biasing layers. When the track width is above 1 xcexcm, this does not render the flux guide unacceptable since a remainder of the width of the flux guide remains relatively soft for responding to field signals from the rotating magnetic disk. Another way of stating the problem is that with submicron track widths the hard bias renders the flux guide with low permeability. Since a flux guide needs a height of approximately 0.25 xcexcm to 0.5 xcexcm the field signal from the rotating magnetic disk is nonexistent or insignificant at the tunnel junction sensor because of the lack of permeability of the flux guide. Accordingly, there is a strong-felt need to provide a submicron track width tunnel junction sensor with a flux guide that has high permeability.
The present invention provides a highly permeable flux guide for a submicron tunnel junction sensor. As background, the tunnel junction sensor is recessed from the ABS and has front and back recessed surfaces. The flux guide has a front surface that forms a portion at the ABS and a back surface that is magnetically coupled to the front surface of the tunnel junction sensor. The flux guide is provided with high permeability by making it an antiparallel (AP) coupled structure. The AP coupled structure includes first and second antiparallel (AP) layers and an antiparallel coupling (APC) layer that is located between and interfaces each of the first and second AP layers. Each of the first and second AP layers has a magnetic moment. Magnetic moments of the AP layers are antiparallel with respect to each other and are parallel to the ABS and the major planes of the first and second AP layers. The magnetic moment of one of the first and second AP layers, such as the second AP layer, has a magnetic moment that is greater than the magnetic moment of the other of the first and second AP layers, such as the first AP layer. The free layer of the tunnel junction sensor has a magnetic moment that is parallel to the magnetic moment of the AP layer which has the greater magnetic moment, such as the second AP layer.
With the present invention hard bias layers on each side of the flux guide are not required in order to stabilize the magnetization of the flux guide. The AP coupled flux guide is more stable than a single layer flux guide without hard biasing since the ends of the AP coupled flux guide have reduced demagnetization. This is because of flux closure between the first and second AP layers. The AP flux guide also has high permeability which means that the flux decay length of the field signal from the rotating magnetic disk can be long which improves the efficiency of the read head. The effective thickness of the AP flux guide is the difference in the thicknesses of the first and second AP layers. For instance, if the first AP layer is 50 xc3x85 thick and the second AP layer is 200 xc3x85 thick the effective thickness is 150 xc3x85. Assuming that the uniaxial anisotropy HK for each layer is 5 xc3x85 the uniaxial anisotropy HK for the AP flux guide can be calculated by the formula HK=(HK1t1+HK2t2)÷(t2xe2x88x92t1). With the above parameters HK=5xc3x9750+5xc3x97200÷200xe2x88x9250=8.2 Oe. Accordingly, the effective uniaxial anisotropy HK of the AP flux guide is 8.2 Oe which renders the AP flux guide relatively soft with high permeability.
An object of the present invention is to provide a submicron track width tunnel junction sensor with a highly permeable flux guide.
Other objects and attendant advantages of the invention will be appreciated upon reading the following description taken together with the accompanying drawings.